The invention disclosed relates to a structure for motor-compressors used with refrigerant fluids.
The component of greatest importance in the majority of refrigeration systems, or conditioning systems generally, is the motor-compressor unit by which the compression of the refrigerant gas is effected.
Such units fall substantially into two different types, one of open design, the other sealed.
The first consists essentially in a compressor, and a motor which drives the compressor, which are embodied as self-contained units and interconnected by a drive coupling. The main advantage of this type of motor-compressor is the possibility of adopting different types of motor to drive the compressor and of gaining easy access to the motor for servicing.
The main drawback, on the other hand, is that of insulating the compressor, since this requires the creation of a hermetic seal around a moving part.
Another disadvantage of this type of compressor is the limited cooling of the motor, which in this instance is externally ventilated, or cooled by a fluid other than that in the refrigeration system.
Sealed motor-compressors, by contrast, are designed such that the compressor affords a housing for the motor, which is therefore encapsulated in a fluid-tight container and cooled by the refrigerant circulating in the system.
Motor-compressors of this type therefore present no problems whatever in terms of creating a hermetic seal; the seal is effected on a static component, since the motor is housed internally of an already fluid-tight container.
There is increased preference for the sealed motor-compressor over the open design type, thanks to the development of new refrigerant fluids, and to the more compact dimensions of such units.
A reduction in the bulk of these sealed units is made possible by virtue of the aforementioned fact that the drive motor is cooled by the self-same refrigerant fluid, which permits of obtaining a practically constant, limited running temperature.
Nonetheless, the fact that the motor is cooled by the refrigerant can occasion additional, serious drawbacks, which arise in the event of malfunction occurring in the motor-compressor. More exactly, overheating in the windings of the electric motor can cause fragments of scorched insulation to be shed, thus contaminating the refrigeration circuit. The high temperatures generated in an overheating electric motor also engender a conversion of the refrigerant and lubricating oil into acids of strong concentration, which increase contamination in the refrigeration circuit.
In smaller machines, such as those for domestic use, this drawback is overcome by replacing the entire refrigeration system, the cost of which is far less than that of effecting repairs. In larger installations, it is impossible even to contemplate a full replacement of the refrigeration system, indeed of the motor-compressor alone, for obvious reasons of cost.
In large refrigeration systems, replacement of the electric motor only is feasible, provided that the refrigeration circuit has not been seriously contaminated, but involves a certain number of operations: the motor-compressor must be drained of refrigerant and the motor removed; the air must be bled off, and the refrigeration circuit must be force dried before it can ultimately be refilled and operated.
The cost of such operations, which is high in any event, becomes even higher due to the fact that it is practically impossible to re-use refrigerant drained from the motor-compressor.
Besides giving rise to problems with servicing and repairs, the difficulty of gaining access to the motor obliges manufacturers and wholesalers to run excessively capacious warehousing facilities in order that the different requirements of users can all be met. In other words, it becomes necessary to maintain a stock of motor-compressor units having different drive motor specifications, in particular, dissimilar in terms of power consumption and power supply -i.e. input voltage and a.c. or d.c. current. In effect, it would be unthinkable to hold stocks of a given number of motor-compressors having certain specifications, and in the light of the foregoing, of a given number of motors with other mechanical or elctrical specifications serving simply to replace those already fitted.
In an attempt to overcome the drawbacks of the sealed motor-compressor unit, while maintaining its advantages substantially intact, the idea has been put forward of accommodating the compressor and the motor in respective compartments that are sealed off hermetically one from the other by a bulkhead which spans the inside of the casing that houses the unit. An example of this expedient is disclosed in Italian Pat. No. 194436, owned by the same applicant.
It has been discovered, nonetheless, that whilst the adoption of this expedient is able to eliminate problems connected with the use of the conventional sealed motor-compressor unit, total isolation of the motor from the compressor and refrigeration system gives rise to fresh difficulties, as regards the attainment of efficient cooling for the motor, and of a degree of insulation that will neither tend to jeopardize the efficiency of the unit, nor result in loss of motor power.
Given that the electric motors most widely used are oil-cooled, an expedient that makes for smoother running of the motor, the option exists of fitting an external heat exchanger to forestall the risk of overheating, thereby raising the cost and increasing the dimensions of the unit. In this case, however, it becomes necessary to limit the rated power of the motor to a certain extent so as to avoid sudden rises in temperature that can damage the motor. Accordingly, the object of the present invention is to set forth a structure for motor-compressor units that will permit of overcoming the aforementioned drawbacks.